1. Field of the Invention
The present invention relates to a positive resist composition, and a method of forming a resist pattern.
Priority is claimed on Japanese Patent Application No. 2008-218288, filed Aug. 27, 2008 and Japanese Patent Application No. 2009-038433, filed Feb. 20, 2009 and Japanese Patent Application No. 2009-185819, filed Aug. 10, 2009, the contents of which are incorporated herein by reference
2. Description of the Related Art
In lithography techniques, for example, a resist film composed of a resist material is formed on a substrate, and the resist film is subjected to selective exposure of radial rays such as light or electron beam through a mask having a predetermined pattern, followed by development, thereby forming a resist pattern having a predetermined shape on the resist film. A resist material in which the exposed portions become soluble in a developing solution is called a positive-type, and a resist material in which the exposed portions become insoluble in a developing solution is called a negative-type.
In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have lead to rapid progress in the field of pattern miniaturization.
Typically, these miniaturization techniques involve shortening the wavelength of the exposure light source. Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers and ArF excimer lasers are now starting to be introduced in mass production. Furthermore, research is also being conducted into lithography techniques that use exposure light source having a wavelength shorter than these excimer lasers, such as F2 excimer lasers, electron beam, extreme ultraviolet radiation (EUV), and X ray.
Resist materials for use with these types of exposure light sources require lithography properties such as a high resolution capable of reproducing patterns of minute dimensions, and a high level of sensitivity to these types of exposure light sources. As a resist material which satisfies these conditions, a chemically amplified resist is used, which includes a base resin that exhibits a changed solubility in an alkali developing solution under action of acid and an acid generator that generates acid upon exposure. For example, a chemically amplified positive resist contains, as a base resin, a resin which exhibits increased solubility in an alkali developing solution under action of acid, and an acid generator. In the formation of a resist pattern, when acid is generated from the acid generator upon exposure, the exposed portions become soluble in developing solution.
Until recently, polyhydroxystyrene (PHS) or derivative resins thereof in which the hydroxyl groups are protected with acid-dissociable, dissolution-inhibiting groups (PHS-based resins), which exhibit high transparency to a KrF excimer laser (248 nm), have been used as the base resin component of chemically amplified resists. However, because PHS-based resins contain aromatic rings such as benzene rings, their transparency is inadequate for light with wavelengths shorter than 248 nm, such as light of 193 nm. Accordingly, chemically amplified resists that use a PHS-based resin as the base resin component suffer from low levels of resolution in processes that use light of 193 nm.
As a result, resins that contain structural units derived from (meth)acrylate esters within the main chain (acrylic resins) are now mainly used as base resins for resists that use ArF excimer laser lithography, as they exhibit excellent transparency in the vicinity of 193 nm (for example, see Japanese Unexamined Patent Application, First Publication No. 2003-241385)
Here, the term “(meth)acrylic acid” is a generic term that includes either or both of acrylic acid having a hydrogen atom bonded to the α-position and methacrylic acid having a methyl group bonded to the α-position. The term “(meth)acrylate ester” is a generic term that includes either or both of the acrylate ester having a hydrogen atom bonded to the α-position and the methacrylate ester having a methyl group bonded to the α-position. The term “(meth)acrylate” is a generic term that includes either or both of the acrylate having a hydrogen atom bonded to the α-position and the methacrylate having a methyl group bonded to the α-position.
These base resins of chemically-amplified photoresists can be obtained by radical polymerization of a plurality of (meth)acrylate ester monomers.
However, the molecular weight distribution (ratio of weight average molecular weight (Mw) to a number average molecular weight (Mn):Mw/Mn] of the base resin obtained by the radical polymerization method exceeds 1.5, which can cause problems such as an inability to adequately control the molecular weight and a low yield.
On the other hand, a chemically-amplified photoresist using a base resin having molecular weight distribution of 1.01 to 1.50 has been proposed (for example, see Japanese Unexamined Patent Application, First Publication No. No. 2003-84436).
However, in recent years, as requirements for miniaturization of resist patterns, further improvement in resolution has been demanded in resist patterns formed by a conventional chemically-amplified photoresist positive composition.
It is an objective to form patterns of minute dimensions of several tens of nanometers in lithography techniques that use electron beams or EUV, and thus it becomes an important issue that resist patterns with high resolution can be formed as the resist patterns have more minute dimensions.